The present invention relates to an immersion nozzle used in continuous casting of molten steel.
In regard to an immersion nozzle used in continuous casting, in the case of billet casting, a straight type immersion nozzle is frequently used to avoid discharged molten steel from colliding with a mold wall at high speed since a distance between a nozzle and the mold wall is short. Further, in the case of slab continuous casting, a bifurcated nozzle having outlet on the narrow side of a mold is used.
In the case of a straight type immersion nozzle, molten steel is discharged mainly in the right downward direction and inclusions and bubbles are accompanied deeply in the mold and therefore, there poses a problem in which inclusions and bubbles are caught in cast steel or liable to deposit on the bent portion at the lower side of the mold to cause a defect. Further, discharged molten steel is mainly directed downward and therefore, temperature drop of molten steel at the meniscus is significant, melting of mold powder becomes insufficient and lubricity between the mold and a solidified shell is deteriorated to thereby cause surface defect of cast steel. In this specification, the meniscus is referred to an interface between molten steel and mold powder in the mold.
Meanwhile, in the case of a bifurcated immersion nozzle, discharged molten steel reaches the narrow side of the mold and thereafter turned back to the nozzle and when an outflow and the turned flow collide with each other, the meniscus is significantly fluctuated and inclusions and bubbles are trapped in cast steel. Further, also in this type of nozzle, there poses a problem in which inclusions and bubbles are deeply accompanied and trapped in cast steel or are deposited on the bent portion at the lower side of the mold. In the case of this type of nozzle, molten steel is discharged from a lower end of an outlet with a particularly high velocity and these problems become further significant in high speed casting since a maximum outlet velocity of molten steel is high. Further, the problem of temperature drop of molten steel at the meniscus is similar to the above-described.
To solve these problems, electromagnetic stirring of molten steel by a magnetic field system has been proposed for the purpose of controlling molten steel flow in the mold. Although controlling of the molten steel flow by electromagnetic stirring is effective, this process cannot be regarded as sufficient countermeasure for high speed continuous casting requested recently. Further, the electromagnetic stirrer is very expensive and the location of installing the system is disposed in a severe environment exposed to high temperature and maintenance and repair of the system is not easy.
In addition to the above-described, as a conventional problem of an immersion nozzle, there causes clogging of the nozzle owing to adhesion of inclusions. This is a problem in which nonmetallic inclusions in molten steel gradually adhere to and deposit on an inner wall of the nozzle, the nozzle finally clogs and cannot be used. Further, even when the clogging is not completed, there is a case in which adhered inclusions is peeled off and trapped into molten steel to thereby causing defect of cast steel.
As a countermeasure against adhesion of inclusions on the inner wall of the nozzle, there has been carried out a method in which inert gas is blown from the inner wall of the nozzle, inclusions in steel are trapped and taken out and are floated up in the mold. However, the method is not regarded as sufficient countermeasure since there is a case in which inclusions gradually adhere onto the inner wall in a sequential continuous casting process, to finally result in clogging of nozzle.
In respect of the problems in the conventional technology mentioned above, there has been requested an immersion nozzle capable of preventing defect factor of cast steel in the mold and preventing adhesion of inclusions on the inner wall of a nozzle to meet request for high quality cast steel and high speed casting.
The inventors have carried out various investigation to provide an immersion nozzle to solve the problems of the conventional technology mentioned above and conceived to provide swirlling to molten steel flow in an immersion nozzle and carried out water model experiments. As a result, it has been found by providing swirling to water flow in a nozzle that an outlet pattern can preferably be controlled such as a reduction in a maximum outlet velocity, uniform discharge from a total of an outlet and this result has been presented (Iron and Steel VoL.80 No.10 P754-758(1994), ISIJ (The Iron and Steel Institute of Japan) International VoL.34 No.11 P883-888(1994)).
In the water model experiment, swirling is provided by installing a swirling blade at an upper portion of the nozzle. A used swirling blade is constituted of a circular disc in a doughnut-like shape having an inner diameter the same as the inner diameter of the nozzle and is provided with 12 of blades each having slope for constituting a swirling flow from water flowed into the nozzle.
The inventors have groped various methods of providing swirling to actual molten steel flow. The shape of a swirling blade used in the water model experiment is complicated, manufacture by a material capable of withstanding molten steel at high temperature has been extremely difficult and the material cannot withstand physical impact of molten steel flow.
Further, a consideration has been given to the fact that swirling motion is provided to molten steel flow in the nozzle by a magnetic field system used in controlling flow of molten steel in the mold. However, it has been impossible to provide swirling to obtain an outflow pattern as in the result of the water model experiment in a short period of time during which molten steel passes in a immersion nozzle.
After all, the inventors have conceived an element which is constructed in a twisted-tape shape which has a simple shape such that it can be manufactured by a material withstanding molten steel flow and which can provide sufficient swirling. With this shape, the element can be manufactured easily and withstand impact of molten steel, further, more or less additional processing after producing and installation thereof in a nozzle are facilitated. Further, the inventors have found that excellent swirling can be provided to molten steel flow in the nozzle by properly setting the twisted-tape shape and completed the present invention.
The present invention is constituted by an immersion nozzle having an element in a twisted-tape shape to provide swirling in molten steel flow in the nozzle. When swirling is provided to molten steel flow in the nozzle by the element in a twisted-tape shape, the molten steel flow in the mold is controlled, a distance of invasion of inclusions and bubbles becomes short and trapping thereof in cast steel is prevented. Further, an effect of preventing inclusions from adhering to an inner wall of the nozzle is also achieved.
According to the present invention, excellent swirling is provided by constituting the shape of the element in a twisted-tape shape such that a ratio L/D of length L and width D falls in a range of 0.5 through 2 and a twisted angle xcex8 is 100xc2x0 or more.
The element in a twisted-tape shape of this invention is applicable to an immersion nozzle of both a straight type and a bifurcated type.
In the case of a straight type immersion nozzle of the present invention, discharge of molten steel is not directed to right downward but mainly in a skew downward direction by which invasion of inclusions and bubbles can be reduced.
Further, when at an outlet of molten steel, an inner wall thereof constitutes a figure of a divergent arc in the vertical section, molten steel flow can preferably be provided in the direction of the meniscus and lowering of temperature of molten steel at the meniscus can be reduced. The effect is further significant when the inner wall of the vertical section constitutes a figure of a divergent arc with a radius of curvature in a range of 30 through 300 mm.
Meanwhile, in the case of the bifurcated immersion nozzle of this invention, the maximum outlet velocity of molten steel can be reduced and therefore, collision of an outflow and a turned flow from the narrow side of the mold is alleviated and meniscus fluctuation can be prevented.
Further, also in the case of the bifurcated immersion nozzle, by constituting an inner wall of a nozzle reaching an outlet in a figure of a divergent arc in respect of the vertical section, molten steel flow in the mold can further preferably be controlled and the temperature drop of molten steel of the meniscus can be reduced. The effect becomes further significant when the inner wall of the vertical section is constituted by a figure of a divergent arc with a radius of curvature in a range of 30 through 300 mm.
According to the present invention, a structure without bottom can be constituted in the bifurcated nozzle which is further preferable in view of preventing adhesion of inclusions.
Further, other embodiment of the present invention is an immersion nozzle having a structure of blowing gas into molten steel flow provided with swirling in the nozzle according to each type of the nozzles mentioned above. According to the gas blowing type immersion nozzle, an effect of trapping and taking out inclusions in molten steel and floating it up in a mold is substantially prolonged.